The invention relates to the storage of natural gas (CNG) or other fluids or gases under pressure in pressure vessels comprised of very long lengths (generally over 1 km and under 30 km) of pipe, e.g., steel pipe. For practical management, these lengths of pipe are coiled on a steel spool by a simple winding process. The result is termed a “Coselle®”. Many Coselles® may be installed in the hold of a ship. A Coselle® carrying ship loads gas in a first port and discharges in a second port. A Coselle® carrying ship may make ˜60 cycles per year. The pressurizing and de-pressurizing associated with the loading and unloading of gas induces major stress changes in the steel of the Coselles®, which could possibly result in fatigue cracking. The long lengths of pipe are necessarily constructed by welding shorter lengths of pipe together. The welds and pipe material adjacent the welds are more sensitive to fatigue problems than the parent metal of the pipe.
It has become apparent during prototype Coselle® testing where 6″ pipe was wound about a hub of ˜9 meters diameter, that higher than average strains are experienced in the outside wall of the pipe at the heat affected zone (HAZ) interface of the circumferential joining welds, i.e., butt welds, contained in the pipe. The highly localized strains, (approximately 6% as opposed to approximately 1.8% average) shorten the fatigue life of the weld and are responsible for limiting the ultimate fatigue life of the Coselle®. As a result, it was necessary to operate at a lower working pressure than would be allowed if the fatigue life were extended, which reduced the gas volume and raised the overall cost of gas transport.
In addition to exceptional strain, the weld properties may also be adversely affected by ovallization of the pipe during the bending process. At the four points of high and low curvature of the oval, stress is increased during the pressurization process and this will reduce fatigue life, particularly for the butt welds.